A variety of processes have been marketed and/or proposed over the years for welding together overlapping portions of a tensioned loop of thermoplastic strap encircling an article. One process employs a heated member to melt a surface layer of each of the strap portions which are then pressed together while the layers merge and cool to form a solidified weld or welded joint. (In the context of this subject matter, the terms "joint," "weld," and "welded joint" are conventionally used interchangeably.)
In a different process, the strap portions are pressed together, and a layer of each strap portion at the interface is melted by means of ultrasonic energy. The layers then cool and solidify while the overlapping strap portions remain pressed together.
Another process is effected by first pressing the overlapping strap portions together and then creating a unidirectional or multidirectional bodily sliding frictional movement between the contacting surfaces of the overlapping strap portions so as to melt the interface region of the overlapping stap portions. The melted interface region is allowed to solidify at rest, but under pressure, so as to bond the overlapping strap portions together.
The last discussed process, which can be generally designated as friction-fusion welding or friction welding, has proven to be especially effective with conventional thermoplastic strap materials such as nylon, polyester, and polypropylene. Such conventional strap is typically provided commercially in widths ranging from about 5 mm. to about 13 mm. and in thicknesses ranging between about 0.25 mm. and about 0.89 mm.
Conventionally produced welded joints in thermoplastic strap have found wide commercial acceptance in many applications. However, a conventional welded joint is typically the weakest part of a tensioned strap loop secured about a package or other object. There is a continuing need for an improved welded joint that has greater strength than conventional welded joints in various types of strap and that can be produced routinely and consistently, and in an economic manner. It would be desirable to produce a welded joint that has a strength that approaches, as close as possible, the tensile strength of the strap.
One aspect of the present invention relates to the provision of a novel strap structure which can be fabricated with certain materials so that when overlapping portions of the strap are welded together, there is produced a welded joint with a unique internal configuration having greater strength.
The above-referenced U.S. patent application Ser. No. 871,693 discloses, inter alia, a welded joint at an interface in overlapping portions of thermoplastic strap wherein the weld is formed by first fusing (melting or liquifying under the application of heat) at least part of the thickness of each strap portion across the width of the interface and then permitting the fused parts of the strap portions to subsequently solidify to form the welded joint.
The weld is created with a selected strap material in a manner such that cavities are encapsulated within the resolidified region of the weld, and the cavities result in the weld having a greater strength. The weld is described in detail in the U.S. Pat. No. 4,707,390 which is assigned to the assignee of the present invention and which is incorporated herein by reference thereto to the extent not inconsistent herewith.
The above-referenced U.S. Pat. No. 4,707,390 and patent application Ser. No. 871,693 each state that the exact mechanism by which the cavities increase the weld strength is not necessarily fully or accurately understood, but nevertheless propose a theory. In particular, it is believed that the improved joint strength of the weld results from the redistribution of stresses within the weld, and that the cavities cause the stress redistribution. It is believed that welds fail when cracks form at the ends of the weld. The cavities are believed to reduce the stresses at a crack tip that is propagating into one or more of the cavities.
The above-referenced U.S. patent application further states that the cavities are thought to result from material in the strap, such as moisture, which forms gaseous bubbles in the molten weld region. The patent application suggests that such material might be added to the weld region environment before or during welding. In particular, the patent application includes the following disclosure:
"It has also been proposed to add such bubble-producing material directly to the surface of strap that does not normally contain such material. For example, conventional polypropylene strap does not hold moisture in amounts sufficient to form bubbles when the strap is welded in accordance with the present invention. However, a layer of a bubble-forming material could be coextruded on each surface of polypropylene strap. Such a layer could be a polyester strap material that normally contains amounts of moisture believed to be sufficient to generate the bubbles during the welding process."
The present invention relates to such additive materials and additional layers of materials which may be incorporated in a strap. Disclosed herein are particular strap structures and additive materials which will facilitate the formation of a strap weld containing encapsulated cavities. The cavities may be alternatively or more broadly characterized as discrete volumes in the resolidified region wherein such discrete volumes could include, but are not limited to including, gaseous bubbles.